System for detecting ice or snow on surface which specularly reflects light

ABSTRACT

A method and apparatus for detecting a presence of a polarization altering substance on a specular surface includes transmitting light to the surface over a transmitting path and receiving the transmitted light from the surface and from the polarization altering substance. An intensity of the light is measured in an optical non-isolator state and in an isolator state before being compared to reference data established for various specular surfaces. The reference data are preferably established by measuring an intensity of the light in both an optical non-isolator state and in an isolator state for a known surface when the polarization altering substance is absent from the known surface.

RELATED APPLICATIONS

This application is a continuation-in-part of PCT application US93/10035filed Oct. 20, 1993, which in turn is a continuation-in-part ofapplication Ser. No. 07/963,840 filed Oct. 20, 1992, now U.S. Pat. No.5,475,370, both of which are assigned to the same assignee as thesubject application.

BACKGROUND OF THE INVENTION

Current airport aviation practices depend on the use of de-icing fluidto remove ice and prevent its future build-up for time periods of 5-10minutes. Verification that wing and other aerodynamic or controlsurfaces are ice free is done visually, often under difficult viewingconditions. Occasionally significant ice build-ups are not noticed, withtragic results. Responsibility for detecting such ice rests with theaircraft crew who rely on visual viewing, perhaps supplemented with anordinary flashlight. Obviously, a need exists for a system which iscapable of accurately and easily determining the presence of ice on anaircraft wing.

Metallic surfaces and dielectric surfaces behave differently whenilluminated with light, particularly with respect to their polarizationproperties. One of the strongest differences and most easily observableis the property of metals to reverse the rotational direction ofcircularly polarized light. For example, the specular reflection ofright handed (clockwise looking towards the source) circularly polarizedlight from a metal surface changes it to left handed (counterclockwise)polarization and vice versa. This effect is used in the construction ofoptical isolators which permit light to initially pass through theisolator but prevent specularly reflected light from returning throughthe isolator back to the source. The optical isolator is a circularpolarizer that is usually implemented from a linear polarizer and aquarter wave retarder plate that has its fast and slow axes located 45°from the polarization axis of the polarizer. The polarizer must precedethe retarder in the light path.

When a metallic surface (or surface painted with a metallic paint), suchas the wing of an aircraft, is illuminated with circularly polarizedlight (which may be generated by passing unpolarized light through acircular polarizer) and the reflected energy viewed through the samecircular polarizer, the resulting image is extremely dim since thecircular polarizer is performing as an isolator with respect to thespecular reflection from the metal surface. Other types of surfaces(birefringent, certain dielectric, matte, etc.) viewed through the samecircular polarizer maintain their normal brightness because uponreflection they destroy the circular polarization. If the circularpolarizer is flipped (reversed) so that the retarder precedes thepolarizer, it no longer acts as an isolator for the illuminating beamand the metallic surface's image will now be viewed of normal (bright)intensity.

Most non-metallic and painted or matte surfaces illuminated withcircularly polarized light and viewed through the same circularpolarizer will maintain their normal intensity. Such surfaces, as wellas a coat of ice on the metal, whether matte white due to a snowcovering or crystal clear due to even freezing will destroy the circularpolarization of the reflected light and therefore take on thedepolarizing property of a matte painted surface with respect to theoptical isolator. A transparent dielectric over metal depolarizescircularly polarized light passing through it if it has numerousinternal point scatterers or is birefringent. Ice has thischaracteristic. Thus, circularly polarized light reflected from apainted surface, snow, ice, or even transparent ice over metal will bedepolarized and will not be affected by the isolator.

Therefore, the image of a clear metal surface that is ice-free willalternate between dark and bright when alternately viewed through anisolator and non-isolator structure, respectively. Apparatus other thanthe combination of optical isolators and non-isolators can produce thesame effect. Any ice or snow covering the metal surface will cause theimage to maintain the same brightness regardless of whether it is viewedthrough an isolator or non-isolator structure or equivalent structures.

SUMMARY OF THE INVENTION

The present invention provides various arrangements for inspecting ametal surface for the presence of ice which compares views of thesurface in an optical isolating and non-isolating manner. Making suchcomparisons in an alternating manner results in the metal surfaceproducing a blinking, on-off, viewing of the reflected light and the iceproducing a steady level of illumination.

In accordance with the invention, various embodiments are provided forinspecting a metallic surface in which there is a comparison orswitching between an optical isolator structure and non-isolatorstructure. In one embodiment, switching is implemented by switching thelight illuminating the metal surface between circularly polarized andnon-circularly polarized light while observing through a circularpolarizing filter of the same hand, i.e., CW or CCW, as required tocomplete the isolator. In another embodiment, the light illuminating thesurface may be kept circularly polarized but viewed alternately througha circular polarizer of the same hand and a non-circular polarizingelement having the same optical attenuation. This is most easilyaccomplished by viewing through the same type of circular polarizerflipped over (reflected light enters the polarizing element first) tokeep it from acting as the circular polarizer to elements of an isolatorwhile simultaneously maintaining the slight light attenuation of itslinear polarizer element.

Another embodiment maintains the illumination in a circularly polarizedstate and alternately views the scene through right handed and lefthanded circular polarizers which will alternately change between theisolating and non-isolating states. A non-isolating state may also beachieved by rotating either the receiver or transmitter quarter waveretarder plate forming a part of the polarizer by 45°. This aligns theslow or fast axis of the retarder with its polarizer. The effect isthat, if done at the transmitter, linearly polarized light passingthrough the quarter wave plate remains linearly polarized and if done atthe receiver, circularly polarized light (which passes through theretarder plate first) emerges linearly polarized at 45° to the originaldirection--it can then pass through the linear polarizer with justslight attenuation.

Rotation of either the transmitter or receiver quarter wave retarder by90° from the position in which it serves to operate as an isolator alsochanges the state to non-isolating because the specularly reflectedcircularly polarized wave is then exactly aligned with the receiverpolarizer as it emerges in the linearly polarized form from thereceiver's quarter wave retarder. Isolating and non-isolating states mayalso be achieved by various combinations of crossed and aligned linearpolarizers, respectively.

Since the reflected light from a specular surface is highly directional,it is beneficial to minimize any change in illumination angle whenchanging from isolator to non-isolator state. Otherwise a change inreflected light intensity caused by a change in illumination angle maybe interpreted as caused by the isolator/non-isolator effect and anerroneous decision made. The high directionality of specular reflectionalso introduces the need to accommodate a large dynamic range ofreceived light intensities. If not properly handled, erroneous decisionscould be made as a result of saturation due to high light levels withinthe receiver.

Background light such as sunlight must be removed from influencing thefinal clear/ice surface decision or the system will be limited tooperation in low light levels. Light reflected from surfaces other thanthe aircraft wing, such as the ground, when viewing downward on the wingmust also be removed in order to provide an unmistakable image of thewing and any patches of ice on it.

Specular surfaces that can be viewed close to the surface normal providea relatively high isolator/non-isolator ratio and therefore a cleardemarcation between the two light levels. However, as the surface isviewed at angles away from normal to the surface, which is necessary fora system viewing from a fixed location, the ratio drops and thereflected light effect becomes closer to that produced by reflectionfrom ice, making it more difficult to reliably distinguish between thetwo.

The preferred embodiment of the invention provides a novel method andapparatus for an enhanced way of determining the surface conditions,that is, the presence or absence of ice or snow. This is accomplished bycomparing the amplitude value of the received reflected signal when theoptical system is in a non-isolating state with the ratio of thenon-isolating state to isolating state amplitude values or the ratio ofthe difference and the sum of these two values. A threshold curvedemarcating the surface clear to ice/snow presence conditions isprovided as a function of the non-isolating state amplitude value andone of the two aforesaid ratios. This curve is generated from data ofreceived signal amplitude values in the non-isolating and isolatingstates with the illuminating source being at different angles relativeto the surface.

Circuits are also provided to process and evaluate the received signalamplitude values in logarithmic form to aid in analysis and to easilyaccommodate for differences in the distance between the illuminatingsource and the surface.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus for detecting the presence of a depolarizing dielectricmaterial, such as ice or snow, on a metal specular reflecting surface.

A further object is to provide a system for detecting ice or snow on themetal (or metallic painted) wing of an aircraft.

An additional object is to provide a system for detecting ice or snow ona metal (or metallic painted) surface which is specularly reflective tolight using circularly or linearly polarized light.

Yet another object is to provide a system for detecting ice or snow on ametal or metallic painted surface in which optical means are used toproduce an on-off light blinking response for a metal surface and asteady light response for any part of the surface covered with ice orsnow.

A further object is to provide a process and apparatus to overcomesystem limitations brought about by ambient light, the large dynamicrange of light intensities, and the poor response of metal surfacesviewed away from a direction normal to the surface.

Still a further object is to provide a novel method and apparatus inwhich the surface condition is determined by comparing the amplitudevalue of the received signal in a non-isolating state of the systemoptics to a ratio of the received signal amplitude values in thenon-isolating and isolating state or a ratio of the difference and thesum of these two values.

Other objects and advantages of the present invention will become moreapparent upon reference to the following specification and annexeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an optical schematic of a circular polarizer with the linearpolarizer facing the illumination source so that the polarizer acts asan optical isolator.

FIG. 1B is an optical schematic of a circular polarizer with the quarterwave plate facing the illumination source so that it passes specularlyreflected light, i.e., it is a non-isolator;

FIG. 2 is an optical schematic of two circular polarizers, one in thetransmit path and one in the detection path, that together form anoptical isolator;

FIG. 3A is a schematic view of an ice detection apparatus based ondirect visual observation using two spot-light illuminators, onepolarized and one not;

FIG. 3B is a schematic view of an ice detection apparatus based ondirect visual observation which uses one circularly polarized lightsource;

FIG. 3C is a detail of the FIG. 3B apparatus for switching the polarizerbetween isolating and non-isolating states in the detection path;

FIG. 4A is a schematic diagram of a video based ice detection systemsuitable for use with high background illumination levels which employstwo strobed light sources;

FIG. 4B is a schematic diagram of a video based ice detecting systememploying one laser based strobed light source suitable for use withhigh background illumination levels;

FIG. 5A is a schematic view of the device used in FIG. 4B to switch thepolarizer from an isolating to a non-isolating state in the detectionpath;

FIG. 5B is a plan view of the motor, polarizer and encoder assembly usedwith the apparatus of FIG. 5A;

FIG. 5C is a schematic view of the photo interrupter device used in theencoder assembly of FIG. 5B;

FIG. 6 is an optical schematic diagram of the laser light source of thesystem of FIG. 4B;

FIG. 7 is a schematic of another embodiment of the invention whichutilizes synchronous detection;

FIG. 8A is a schematic view of an embodiment which uses two videocameras and a beam splitter device;

FIG. 8B is a schematic view of the optical path of FIG. A using twomirrors to replace the beam splitter device of FIG. 8A;

FIG. 9A is a schematic view of a polarization sensitive camera basedupon a variation of color camera technology which is particularlysuitable for use in the receive path;

FIG. 9B is a section view of details of the polarization sensitivecamera;

FIG. 10a is a side view of an ice detection system lightprojector/receiver viewing an aircraft wing;

FIG. 10b is a front view of the light projector/receiver;

FIG. 10c is a front detail view of one light source;

FIG. 10d is a schematic view of the receiver portion of the icedetection system;

FIG. 10e is an alternate version of the receiver portion of the icedetection system using alternating quarter wave retarder plates;

FIG. 10f is an alternate version of the receiver portion of the icedetection system using alternating linear polarizer plates;

FIG. 11 schematically depicts the view coverage of the system;

FIG. 12 is a schematic of an alternative light source producing coaxialbeams for isolator and non-isolator states;

FIG. 13 is a schematic of a scanning narrow beam ice detection system;

FIG. 14 depicts the time relationship of waveforms within the system asa function of surface distance;

FIG. 15 is a graph depicting the receiver input ratio as a function ofangle from surface normal for surfaces with different properties;

FIG. 16A, 16B and 16C together comprise a flow diagram of the processfor categorizing the measured values as clear, ice, and non-ice;

FIG. 17a shows the waveforms for an analog system for determiningleading edge delay;

FIG. 17b shows the waveforms for a digital system for determiningleading edge delay;

FIG. 18 is a graph plotting the non-isolator/isolator state receivedsignal amplitude value ratio against the non-isolator state value;

FIG. 19 is a schematic diagram of a circuit for implementing the surfacecondition determining condition in accordance with FIG. 18;

FIG. 20 is a graph plotting the ratio of the sum and differences of thenon-isolator and isolator data received signal amplitude values againstthe non-isolator state value; and

FIG. 21 is a schematic diagram of a circuit for implementing the surfacedetermining conditions in accordance with FIG. 20.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates the operation of a circular polarizer used as anisolator. Light is emitted from an unpolarized light source 13, whichpreferably is as close to monochromatic as possible. The light is shownas unpolarized by the arrows in two orthogonal directions along ray path20, the path the light is following. The unpolarized light passesthrough a linear polarizer 11 which has a vertical polarization axis.The light passing through linear polarizer 11 takes ray path 21, alongthis path illustrated as vertical polarization by the double arrow.

The vertically polarized light at ray path 21 passes through a quarterwave retarder plate 12. The retarder 12 is a plate made frombirefringent material, such as mica or crystal quartz. Its purpose is tochange linearly polarized light from polarizer 11 into circularlypolarized light. Any ray incident normal to the retarder plate 12 can bethought of as two rays, one polarized parallel to the parent crystal'soptic axis (e-ray) and the other perpendicular (o-ray). The e-ray ando-ray travel through the plate 12 at different speeds due to thedifferent refractive indices. The plate 12 is said to have a "fast" anda "slow" axis.

The quarter wave retarder plate 12 has its slow and fast axes both at45° relative to the vertical axis of the linear polarizer 11 so that theemerging circularly polarized light from plate 12 along ray path 22 isrotating in a CCW direction as viewed facing the light source from areflecting surface 14. A metallic surface, which is a specularreflector, and a dielectric surface, i.e., ice or snow, behavedifferently when illuminated with light, particularly with respect totheir polarization properties. A strong and easily observable differenceis the ability of a metal to reverse the rotational direction ofincident circularly polarized light. The specular reflection ofright-handed (CW) circularly polarized light from a metal surfacechanges into left-handed (CCW) polarization and vice versa.

This effect is used in the construction of optical isolators whichpermit light to initially pass through the isolator but prevent suchlight when specularly reflected from returning through the isolator backto the light source. When the optical isolator is a circular polarizerit is usually implemented from a linear polarizer and a quarter waveretarder plate that has its fast and slow axes located 45° from thepolarization axis of the polarizer.

In FIG. 1A, surface 14, which is a specular surface, reflects theincident circular polarized light back along ray path 23. The reflectedlight continues to rotate as viewed from the surface 14 in the CCWdirection but has now changed "hand", in terms of right hand and lefthand, because it is rotating in the same direction with its direction oftravel changed.

The reflected light on ray path 23 passes through the quarter waveretarder 12 and emerges no longer circularly polarized but linearlypolarized in the horizontal direction, which is shown along ray path 24.Because the light at ray path 24 is horizontally polarized it is notpassed by the (vertical) linear polarizer 11. Therefore, none of thespecularly reflected light gets through to ray path segment 25 to enterthe eye 26, which is shown near the location of the light source 13.Thus, the quarter wave retarder plate 12 acts as an optical isolator.That is, light from the light source 13 is passed through the circularpolarizer and reflected by the specular surface 14 but cannot passthrough the circular polarizer back in the other direction and so isblocked before it gets to the eye.

FIG. 1B shows the same quarter wave plate and linear polarizercombination used, but the sequence of the elements is reversed. Here,the quarter wave retarder plate 12 is facing the light source 13 and thelinear polarizer 11 is facing the output side towards the reflectingsurface 14. The light rays now emerge from light source 13 in anunpolarized form along ray path 20 and pass through the quarter waveplate 12. However, because the light is not polarized the quarter waveplate 12 does not change any polarization properties. The light thenpasses through the linear polarizer 11 and becomes vertically polarizedalong ray path 22.

Surface 14 specularly reflects with the same polarization the verticallypolarized light which travels along ray path 23 back towards the linearpolarizer 11 with the same polarization. The light now enters thequarter wave retarder plate 12. Because the light entering plate 12 ispolarized in the vertical direction, it emerges from the quarter waveplate circularly polarized. However, this is of no consequence to theeye 26, so the eye sees the light that has been reflected from thesurface 14. Thus, in this case with the light first entering the quarterwave plate 12 and then passing through the linear polarizer 11 and beingspecularly reflected back to the eye through the linear polarizer andthe quarter wave plate, there is little loss in the light intensity.

As can be seen in the comparison of FIGS. 1A and 1B, light from the samelight source 13 reflected from the specular reflection surface 14 isviewed by the eye 26 either dim or bright depending upon the location ofthe quarter wave retarder plate 12 relative to the linear polarizer 11.That is, FIG. 1A effectively is an optical isolator while FIG. 1B is anon-isolator.

FIG. 2 shows the same implementation of a circular polarizer as in FIG.1A, with the receive path and the transmit path each having their owncircular polarizers 30 and 40. Both circular polarizers are in the sameorder. That is, both linear polarizers 11a and 11b are on the left, oneadjacent to the light source 13 and the other the eye 26, and bothquarter wave retarders 12a, 12b are on the right adjacent to thereflective surface. Thus, as shown, the light from light source 13enters the linear polarizer 11a, exits vertically polarized, passesthrough the quarter wave plate 12a and emerges rotating CCW as viewedfrom the specular reflecting surface 14. The light reflects off thesurface 14 still polarized rotating CCW as viewed from surface 14 andpasses through the circular polarizer 40 in the return direction path toenter quarter wave plate 12b, from which it exits horizontally polarizedto the vertical linear polarizer 11b which blocks the light. Linearpolarizer 11b in the reception leg is distinct and separate from thelinear (vertical) polarizer 11a that was used in the transmit leg.Because the polarization of the light ray along ray path 24 ishorizontal, the light does not pass through the linear polarizer 11b andcannot enter the eye 26.

When a metallic surface, such as the wing of an aircraft, is illuminatedwith circularly polarized light produced by the device of FIG. 1A andthe reflected energy viewed through the same circular polarizer theresulting image is extremely dim since the circular polarizer isperforming as an isolator with respect to the specular reflection of thecircularly polarized light (of opposite hand) from the metallic surface.

A painted portion (non-specular) of the surface illuminated withcircularly polarized light does not reflect light in a polarized form.Instead, it destroys the circular polarization and makes the reflectedlight unpolarized. Thus, the unpolarized light reflected from a paintedsurface portion when viewed through the same circular polarizer of FIG.1A will maintain its normal intensity. The same holds true for circularpolarized light reflected from a wing covered by ice or snow. However,other common harmless substances such as water or de-icing fluid thatmay be on the wing do not destroy the circular polarization of thereflected light.

As explained with respect to FIG. 1B, the components of the circularpolarizer of FIG. 1A are flipped (rotated) such that the retarder plate12 precedes the linear polarizer 11 with respect to the light source 13,so it no longer acts as a circular polarizer to an illuminating beam.Accordingly, the reflection of circular polarized light from the metalsurface will pass back to the eye and will be of normal (bright)intensity. The image intensity of such light reflected from a painted ordielectric (non-specular) surface also will be unchanged as in theprevious case.

When a metallic surface is alternately illuminated and viewed by theisolator and non-isolator devices of FIGS. 1A and 1B, the return imagesat the eye 26 will alternate between dark and bright. A painted ordielectric non-specular surface will remain uniformly bright to thealternation since the light reflected from the painted or dielectricsurface is not polarized and will not be isolated.

Assuming that a metallic surface has a patch of ice thereon or is coatedwith ice, the ice being either matte white due to snow covering orcrystal clear due to even freezing, the circular polarization of thereflected light is destroyed and therefore takes on the property of amatte painted surface with respect to the optical isolator. That is,referring to FIG. 1A, if there is ice on any portion of the specularsurface 14, then the circularly polarized light along ray path 22impinging upon such portion of the surface will not have itspolarization reversed. Instead, it will have the effect of a paintedsurface so that the returned light will be non-polarized and will passto the eye, i.e., the returned image will be bright.

Accordingly, upon alternately illuminating and viewing an ice-freemetallic surface 14 with the circular polarizer-isolator of FIG. 1A andthe non-isolator of FIG. 1B, the return viewed by the eye 26 willalternate between dark and bright respectively. Any ice or snow coveringa portion of the metal surface 14 will cause that portion of the imageto maintain the same brightness regardless of whether it is viewedthrough an isolator or non-isolator structure upon such alternateillumination and viewing.

Switching between an isolator structure, e.g., FIG. 1A, and non-isolatorstructure, e.g., FIG. 1B, may be implemented by switching the lightilluminating the metallic surface between circularly polarized andnon-circularly polarized light while observing through a circularpolarizing filter of the same hand, i.e., CW or CCW, as required tocomplete the isolator. As an alternative, the light illuminating themetallic surface may be kept circularly polarized but viewed alternatelythrough a circular polarizer of the same hand and a non-circularpolarizing element having the same optical attenuation. This is mosteasily accomplished by viewing through the same type of circularpolarizer flipped over (reflected light enters the polarizer elementfirst) to keep it from acting as the circular polarizer to elements ofan isolator while simultaneously maintaining the slight lightattenuation of its elements.

Another arrangement is to maintain the illumination in a circularlypolarized state. Thereafter, the surface would alternately be viewedthrough right-handed and left-handed circular polarizers whichalternately change between the isolating and non-isolating states.

A non-isolating state also may be achieved by rotating either thereceiver or transmitter quarter wave retarder plate 12 by 45°. Thisaligns the slow or fast axis of the retarder with its linear polarizer.The net effect is that, if done at the transmitter, linearly polarizedlight passing through the quarter wave plate remains linearly polarized.If done at the receiver, circularly polarized light (which passesthrough the retarder plate first) emerges linearly polarized at 45° tothe original direction. It can then pass through the linear polarizer tobe viewed with just slight attenuation.

Rotating either the transmitter or receiver quarter wave retarder by 90°from the position in which it serves to operate as an isolator alsochanges the state to non-isolating because the specularly reflectedcircularly polarized wave is then exactly aligned with the receiverpolarizer as it emerges in linearly polarized form from the receiver'squarter wave retarder.

The following table illustrates the implementation that may be used whenalternating either the illumination (transmitter) or receiver (detector)polarizing elements or vice versa to change the overall path from anisolator to a non-isolator structure:

    ______________________________________                                        Between Transmitter                                                                          Between Surface                                                and Surface (or                                                                              and Receiver (or                                               Surface and Receiver)                                                                        Transmitter and Surface)                                       ______________________________________                                        CW only        [CW, LP] [CW, UP] [CW, CCW]                                    CW, LP         CW                                                             CW, UP         CW                                                             CW, CCW        CW or CCW                                                      LP             LP+, LP-                                                       LP, UP         LP-                                                            ______________________________________                                    

In the table above the following abbreviations are used;

CW Clockwise polarization--(Right handed)

CCW Counter Clockwise polarization--(Left handed)

LP Linear polarization

LP+ Linear polarizer aligned with LP

LP- Linear polarizer at blocking angle (e.g. 90°) to LP

UP Unpolarized

Alternating states are separated by commas.

Equivalent sets of alternating states are isolated by square brackets.

In any row CW and CCW may be interchanged. In any row CW may be replacedby RH (right hand) and CCW by LH (left hand).

The columns can be interchanged, i.e., the action can be either on thetransmitter or receiver leg.

The table shows that when using linear polarization the isolating staterefers to the receiving polarizer being orthogonal to the polarizationof the transmitted energy beam and the non-isolating state refers to anyof the following conditions:

a) non-polarized transmission

b) no polarizer in receiver path

c) polarizer in receiving path is approximately aligned with thepolarization of the transmitted beam.

FIG. 3A is a schematic view of a monocular version of an ice detectionsystem suitable for night use based on direct visual observation. Thedirect visual observation receiver uses a non-inverting telescope 50with a circular polarizer 40, like the circular polarizer of FIG. 1A, atits entrance. Two spotlights 13a, 13b are used for the source ofillumination, i.e., the transmitter. One spotlight 13a has a circularpolarizer 30 isolator, like FIG. 1A, mounted to it. The other spotlight13b has a neutral density filter such as a "same hand" circularlypolarized filter 30a mounted backwards so that the light coming throughis linearly and not circularly polarized, i.e., like the non-isolator ofFIG. 1B.

The two spotlights 13a, 13b illuminate a common over-lapping area of asurface shown as the entire area or a portion of an aircraft wing 15having a patch 16 of ice thereon. The clear (no ice) portions of thewing 15 form a specular reflecting surface such as the surface 14 ofFIGS. 1A and 1B. The wing 15 is observed by the field of view 23 of thenon-inverting telescope 50. Both spotlights 13a, 13b and thenon-inverting telescope 50 are mounted on a support structure 52, whichin turn is mounted to a tripod or boom 54. A power supply and sequencer51 for the spotlights 13a, 13b is also located on the tripod boomstructure. Two outputs from the sequencer 51 are taken along wires 53aand 53b to connect with and alternately energize the spotlights 13a and13b, respectively.

The eye 26 is shown looking through the telescope 50. The field of viewof the upper spotlight 13a is shown as fan 22a and that of the lowerspotlight 13b as fan 22b. The region observed by the non-invertingtelescope 50 is formed from the fan of rays reflected back from wing 15into the telescope 50 within field of view 23.

In operation, the sequencer 51 alternates between sending a voltage toand alternately energizing spotlight 13a and then spotlight 13b duringcorresponding time periods "a" and "b". When the voltage is applied tospotlight 13a the outgoing light is circularly polarized by polarizer 30and the light emerges in fan 22a which illuminates the aircraft wingsurface 15. The light from fan 22a reflected from the aircraft wing 15passes back through field of view 23 into the circular polarizer 40 ofnon-inverting telescope 50 where it may be viewed by the eye 26 duringthe interval "a". During the period "a" an optical isolator arrangementis in place because there are two circular polarizers 30 and 40 in thepath. This is shown in FIG. 2. That is, metal areas of the wing whichproduce a specular mirror like reflection reverse the "hand" of theincident circularly polarized light and prevent it from passing backthrough the isolator. Therefore, the eye 26 sees a very dark regioncovering the aircraft wing, except where there is ice, which is shown onpatch 16 of the aircraft wing and which area will show brighter to theeye through the telescope.

When spotlight 13a is turned off and spotlight 13b is turned on duringperiod "b", the light emerging from spotlight 13b is not circularlypolarized. Now the reflection coming back to telescope 50 from both theareas with ice or a metal area without ice will approximately maintaintheir normal brightness. Thus as the sequencer 51 alternately energizesthe spotlights 13a and 13b, the image at the eye 26 from any area thatis metal, specular, and ice free will appear to blink on and off. Thiswill be "on" (bright) when the optical isolator is not in operation and"off" (dark) when isolation exists. However, areas that have ice willnot blink and will have essentially constant brightness, because thepolarized light produced during period "a" is depolarized upon impingingand being reflected from the ice or the metal under the ice.

FIGS. 10a-10f illustrates a concentric arrangement of illuminating lightsources 13 surrounding a camera 80 all mounted in a support structure 52forming an ice detection system suitable for daylight use. FIG. 10ashows a side view of structure 52 with light beams in fans 22a and 22baimed toward aircraft wing 15 with a patch of ice 16. Reflected lightfrom surfaces of wing 15 and patch 16 are collected within the field ofview 23 of camera 80 mounted on structure 52. Light sources 13 containsources of linearly polarized light that project light through quarterwave retarder windows 31a and 31b shown in side edge view in FIG. 10a toproduce beams in fans 22a, 22b. Windows 31a and 31b are tilted away fromthe optic axes of beams 22a and 22b respectively, in order to preventlight reflections from aircraft surfaces 15 (metal) and 16 (ice) frombeing redirected from windows 31a, 30b back towards surfaces 15 and 16.

The combination of linearly polarized light from sources 13 and quarterwave retarder windows 31a, 30b produce circularly polarized light beamsin fans 22a and 22b. Reflected light from the non-ice covered surface ofwing 15 is circularly polarized whereas light reflected from ice patch16 is substantially unpolarized.

FIG. 10b shows a front view of structure 52 with six light sources13a-13f surrounding camera 80. Each light source 13 is similar inconstruction and has an array of linearly polarized light sources behindquarter wave retarder window 31a. As shown in FIG. 10c, each lightsource 13 has four similar segments 203-1 through 203-4. Each segment203 has two assemblies 203a and 203b that have a plurality of linearlypolarized light sources 103. Each light source 103 is, for example, anLED (such as AND180CRP which produces an 8° beam) mounted behind alinear polarization filter. The polarization axis of the filter ispreferably oriented at 45° to the vertical to minimize preferentialreflection of the light from the surface 15 to be illuminated. Thepolarization axes of the filters on assemblies 203a and 203b are mountedorthogonal to each other to provide the basis for distinguishing areascontaining ice such as patch 16 on wing surface 15.

Assemblies 203a and 203b are placed in close proximity to assure thatthe light projected from each impinge on wing surface 15 at nearly equalangles of incidence to minimize differences in reflected energy and toprovide the largest possible depth over which the beams of light fromeach assembly 203a and 203b coincide. Assemblies 203a and 203b arepreferably independently adjustable in two directions to enablealignment of the light beams from these assemblies. They can each betilted vertically and rotated horizontally by amounts to provide overlapof the beams from light sources 13a through 3f.

FIG. 10d shows camera 80 with associated quarter wave retarder plate 12to convert circularly polarized light received within camera view angle23 to linearly polarized light that can be blocked by linear polarizer11 (when the polarization is orthogonal to the polarization axis).

During operation of the system of FIGS. 10a-10d, all assemblies 203a oflight sources 13 are strobed to produce light circularly polarized ofone hand. Then when camera 80 is ready to record the next image, allassemblies 203b of light sources 13 are strobed to produce lightcircularly polarized of the other hand. Specularly reflected light fromthe metal part of wing surface 15 will be rejected in one image and notin the other, creating a blinking effect. Light reflected from ice patch16, however, will be substantially non-polarized and recorded in allimages formed within camera 80 without a blinking effect.

The images can then be directly displayed on a TV monitor for anobserver to determine if any ice is present. The images also can beprocessed and presented to an observer as enhanced images clearlydefining areas where ice patch 16 is present on wing surface 15.

An alternate arrangement to that shown in FIGS. 10a-10d is to omitquarter wave retarding plate windows 30a and 30b from the light sources13, and quarter wave retarding plate 12 from camera 80. Linear polarizer11 will reject light reflected from clear portions of surface 15 whenthe projected polarization is orthogonal to its polarized axis and passlight from the same surface when the projected polarization is alignedto its polarization axis.

A further alternative arrangement is to use a single linear polarizationfilter (or separate filters with their polarization axes aligned) infront of light sources 103 on assemblies 203a and 203b. FIG. 10eprovides a detail of camera 80 surrounded by rectangular quarter waveretarder plate segments 12a and 12b bent into half-cylinders to form abarrel 61 that is rotated to alternate the two half-cylinder segments12a and 12b to be in front of camera 80 when light source 13 is strobed.One quarter wave retarder plate segment 12a is arranged to acceptcircularly polarized light of one hand received in camera view angle 23and convert it to linear polarization aligned to linear polarizer 11,thus allowing the light to pass to camera 80. The other quarter waveretarder plate segment 12b is arranged to accept the same light andconvert it to linear polarization orthogonal to the polarization axis oflinear polarizer 11, thus preventing the light from reaching camera 80.

Using the camera of FIG. 10e, all assemblies 203a and 203b of the lightsources 13 are strobed simultaneously when camera 80 is ready to recordan image. Barrel 61 is rotated to synchronize the positions of segments12a and 12b to be present alternately for sequential images. Lightsources 13 project circularly polarized light of one hand on a wingsurface 15. Light reflected from specular portions of wing surface 15will be circularly polarized and produce a blinking image in camera 80as segments 12a and 12b alternately cause the reflected light to bepassed or blocked respectively. Unpolarized reflected light from icepatches 16 will pass to camera 80 when either segment 12a or 12b ispresent and the images will not blink. Further, since the closely spacedlight source assemblies 203a and 203b are strobed for both images, thereis no angular shift in the light source that could alter the reflectedlight intensity. This produces less light intensity variation fromspecular surfaces that could tend to reduce the blinking effect.

A further alternate arrangement omits the quarter wave retarding platewindows 31a and 31b from light sources 13. FIG. 10f shows video camera80 surrounded by rectangular linear polarizer segments 11a and 11b bentinto half-cylinders to form a barrel 61 that is rotated to alternate thetwo segments previously described. Segment 11a is arranged to have itspolarization axis aligned to the polarization received in camera viewangle 23, reflected from wing surface 15. Segment 11b is arranged tohave its polarization axis orthogonal to the same light.

Using the camera of FIG. 10f all assembles 203a and 203b of the lightsources 13 are strobed simultaneously and produce linearly polarizedlight of one polarization when camera 80 is ready to record an image.Barrel 61 is rotated to synchronize the positions of segments 11a and11b to be present alternately for sequential images. Light reflectedfrom specular portions of wing surface 15 will be linearly polarized andproduce a blinking image in camera 80 as segments 11a and 11balternatively cause the reflected light to be passed or blockedrespectively. Unpolarized reflected light from ice patches 16 willpartially pass to camera 80 when either segment 11a or 11b is presentand the images will not blink.

A further alternative retains the quarter wave retarding plate windows31a and 31b of the light sources, and adds quarter wave retarder plate12 shown in FIG. 10f. This produces circularly polarized light of onehand from light sources 13 for projection upon wing surface 15. Plate 12then converts the circularly polarized light from specular portions ofwing surface 15 to linearly polarized light aligned to the polarizationaxis of segment 11a and orthogonal to the polarization axis of segment11b. Thus a blinking image will be produced for specular portions ofwing surface 15, but not for iced patches 16 that reflect light that issubstantially unpolarized.

FIG. 11 is a detail of the coverage of the ice detection system of FIGS.10a-10f. Light beams in fans 22a and 22b from light sources 13 are aimedto illuminate surfaces at distances between points 35 and 36 from camera80 and within camera view angle 23. This is done in both the verticaland horizontal planes.

Another form of light source 13 of FIGS. 10a-10f is shown in FIG. 12. Alamp 33a, such as a flash lamp, has its light shaped into a beam 20a byoptics. Beam 20a is linearly polarized by polarizer 11a forming linearlypolarized beam 21a whose polarization is aligned to the polarizationaxis of polarizing beam combiner 603. Polarizing beam combiner 603 is apolarizing beam splitter such as Melles Griot 03PBS049 which is used inreverse to form a beam combiner. Beam 21a passes through beam combiner603 and quarter wave retarder plate 12 to form circularly polarized beam22. Likewise lamp 33b forms beam 20b, which passes through linearpolarizer 11b forming linearly polarized beam 21b whose polarization isorthogonal to the polarization axis of beam combiner 603. Beam 21b isturned 90°, passes through plate 12 and can be aligned to coincide withbeam 21a in beam 22. Since the two beams 21a, 21b coincide in beam 22,any angular offset effects of the light source are avoided as the lamps33a and 33b are alternately strobed for reception by a camera 80 asshown in FIG. 10d. The principles of operation are as previouslydescribed, with lamp 33a of FIG. 12 replacing assembly 203a of FIG. 10cand lamp 53b replacing assembly 203b. Alternately, quarter wave plates12 in FIG. 12 and FIG. 10d may be discarded to produce a system relyingon linearly polarized light rather than circularly polarized light aspreviously described.

The linear polarizers 11a and 11b in FIG. 12 are not required if anabsorber 17 is placed as shown to absorb the orthogonal polarized lightof beam 21a, which will be reflected by beam combiner 603, and thealigned polarized light of beam 21b, which will pass through beamcombiner 603.

The intensity of light reflected from a specular surface can vary bymany orders of magnitude depending on the viewing angle. If camera 80has an insufficient dynamic range, specular reflections of highintensity can cause saturation in portions of the image that can spreadto adjacent areas providing a distorted image and may obscure theblinking effect, preventing the proper system operation.

One solution to the dynamic range problem is to first form theblocking/non-blocking image pairs with a normal level of light intensityand then with a lower level projected light intensity. The overalldynamic range of the system is thus increased by the amount (say 10:1)of the light reduction. If a greater dynamic range is required, a thirdimage pair can be made at a further reduction. The overall dynamic rangeof the system is thus increased by the product of the reductions (say10×10=100:1).

The reduction of the amount of light projected by light source 13 doesnot improve on the interference introduced by background light energythat may be within the field of view and adds to the image. A preferredmethod of increasing dynamic range, while at the same time reducingbackground interference, is to take several blocking/non-blocking imagepairs where each pair is taken at a reduced camera sensitivity byreducing the aperture size or aperture time of camera 80 while keepingthe projected light intensity constant from light source 13. Backgroundinterference can be further reduced by filtering the light received bycamera 80 with a filter transparent to the wavelengths of lightprojected by light source 13 and blocking background light of otherwavelengths. Because a sensor's response to light may be non-linear andthe ratios desired are not necessarily constant, it may not be possibleto merely subtract the observed value of background light. Rather, thebackground light and the non-isolator light may be used as an index (orindices) into a look-up table of predetermined values to serve as athreshold for determining if the isolator light level is an indicationof the presence of ice. The method for automated processing is describedbelow.

FIG. 3B shows another ice detection apparatus especially suitable fornight use, which is based on direct visual observation and uses only onelight source 13 with a circular polarizer 30, such as in FIG. 1A. InFIG. 3B the receiver telescope 50 has apparatus at its input forchanging a circular polarizer between the isolating (FIG. 1A) andnon-isolating (FIG. 1B) states. Here, the light source 13 and thetelescope 50 are mounted on a support structure 52 of a boom mount ortripod 68. A power supply 67 for light source 13 also is mounted on theboom.

Power supply 67 supplies the power to light source 13 along cable 66b.Light source 13 incorporates a circular polarizer 30, such as that ofFIG. 1A. The field illuminated by light source 13 is shown as fan 22aand encompasses an aircraft wing area 15 which has a patch of ice 16.Telescope 50 has a field of view encompassing the aircraft wing, orportion of the wing, and this is shown as the rays in field of view 23which enter the telescope 50. Telescope 50 alternates between opticalisolation and non-isolation to the reflected light using a circularpolarizer made of a fixed linear polarizer 41 and quarter wave retarderplate 42a. As shown in FIG. 3C, the quarter wave retarder plate 42a isrotated about its optical axis by drive 65.

Referring also to FIG. 3C, the apparatus for rotating the quarter waveretarder plate 42a is shown. The quarter wave retarder plate is rimdriven by friction drive 65 attached to a motor shaft 64 driven by amotor 63 which itself is attached to telescope housing 61. Bearings 62between the quarter wave retarder plate 42a and the housing 61 relievefriction so that the quarter wave retarder plate 42a may freely rotateabout its optical axis. When the quarter wave retarder plate 42a hasrotated to such a position that its slow and fast axes are at 45° to thevertical, as shown in FIG. 2, the unit acts as an optical isolator andany circularly polarized light that is specularly reflected from theaircraft wing 15 cannot pass through the combination of the quarter waveretarder and the linear polarizer to the eye 26.

A similar end may be achieved by rotating the linear polarizer 41 viarim drive 60 and keeping the quarter wave retarder plate 42a fixed or bykeeping both linear polarizer 41 and quarter wave retarder plate 42afixed and rotating a half wave plate mounted between them with rim drive60.

The position for optical isolation is achieved twice during twopositions spaced 180° apart of each revolution of the quarter waveretarder 42a. At any other position of rotation of plate 42a, there isno isolation and the circularly polarized light reflected from thevarious portions of the wing, both metal and ice, is free to passthrough to the eye with only minimal attenuation. Therefore, thespecularly reflective metal portion of the wing that is not covered withice will reflect light from the light source 13, circularly polarized,back through the isolating mechanism, linear polarizer 41 and quarterwave retarder plate 42a, and this specularly reflected light will beinterrupted twice per revolution and blink off completely. During theother positions of the circular polarizer retarder plate 42a rotationthe light will pass through to the eye 26. Thus, the "on"-"off" blinkingeffect will be produced twice for each rotation of plate 42a.

On the areas of the wing 15 when there is ice present, the incidentcircularly polarized light from light source 13 and polarizer 30 will bedepolarized due to the surface of the ice or by passing through the ice.This depolarized light will pass through the isolator, liner polarizer41 and quarter wave retarder plate 42a at the telescope 50 regardless ofthe rotational position of the quarter wave retarder plate 42a. That is,even when the plate 42a is in one of its two isolating positionsrelative to reflected polarized light, the non-polarized light reflectedfrom the ice will pass through to the telescope as well as when theretarder plate is in a non-isolating position.

The eye 26, which is looking through the telescope 50, is able todifferentiate between the blinking effect produced by the ice freesection of the wing 15 and the non-blinking effect produced by patches16 of the wing with ice. That is, the sections of the wing covered withice 16 will appear to have constant illumination and the ice freesections of the wing will appear to blink at a rate of twice the speedof rotation of the quarter wave plate 42a.

In either of the embodiments of FIGS. 3A and 3B, the apparatus can bemoved to scan all parts of the wing if the field of view is not largeenough.

FIG. 4A shows an indirect viewing video-based ice detecting system thatemploys two strobe lamp spotlights and is suitable for use with highbackground illumination levels.

The system of FIG. 4A is similar to that of FIG. 3A in that it employstwo strobe lamps 93a, 93b. These lamps are of the type which produce ahigh intensity output, for example a xenon lamp, for a short timeperiod. Here, both strobe lamps 93a and 93b have circular polarizers,such as in FIG. 1A, attached. One is a right handed circular polarizer30a and the other a left handed circular polarizer 30b. The strobe lamps93a and 93b are used in conjunction with a conventional video camera 80with a lens 81 having a right handed circular polarizer 40 at its input.

The analog signals of the image produced by the video camera, whichobserves the scene illuminated by the strobe lamps 93a, 93b, are sent toa conventional frame grabber 70. The frame grabber 70 converts theanalog video signal from camera 80 to digital form and stores the datain a digital memory buffer 70a. Pulse generator 75 is used to initiatethe strobing of the lights and the grabbing of a single isolated frameby the frame grabber from the video camera.

The system also preferably has a digital to analog converter and syncgenerator so that the image stored in the buffer 70a can be sent fromthe frame grabber video output to a video monitor or VCR 72 along cable71. The video monitor and the video cassette recorder (VCR) arecommercially available. As an alternative, the video monitor may have adisk recorder which is also commercially available. The frame grabbermay be purchased with additional memory attached and a computer as partof one single image processor unit. Frame grabber 70 and its memory,plus the computer, may be bought commercially as the Cognex 4400.

A flip flop 85 alternates between states on every strobe pulse producedby pulse generator 75. This allows selectively gating a strobe pulse toeither lamp 93a or 93b so that they are illuminated alternately. When apulse trigger input is received by the frame grabber 70 from pulsegenerator output 76, a camera synchronized strobe pulse is generatedwhich is fed from the frame grabber output 74 to the flip flop 85. Thestrobe pulse toggles flip flop 85 and is also gated through one of twoAND gates 89a and 89b. When the flip flop 85 is in one state the strobeout of the frame grabber is gated through AND gate 89a to the input 94aof strobe lamp 93a along wire 53a. When flip flop 85 is in its otherstate, a pulse is sent along wire 53b to input 94b of strobe lamp 93b.Thus, lamps 93a and 93b are alternately illuminated.

The field of view from the strobe lamp 93a with right hand circularpolarizer 30a is shown as fan 22a. The illumination area from strobelamp 93b with left handed circular polarizer 30b is shown as fan 22b.The video camera 80 has a field of view 23 that covers the overlappingregion between fans 22a and 22b. In the video camera field of view 23are the wing 15 with ice patch 16. The images that correspond to wing 15and ice patch 16 that are shown on the video monitor 72 are labeledcorrespondingly as wing image 15a and ice image 16a.

During operation, the pulse generator 75 is set to provide triggersignals at a constant rate, e.g., in a range between 1 and 10 Hz. When atrigger signal enters the frame grabber input 77, it is synchronizedwith the frame grabber internal cycle and at the proper time the framegrabber provides a strobe to flip flop 85 which is passed on to strobelamps 93a or 93b. The strobe output is timed to be properly aligned withthe frame synchronization signal that is sent along cable 84 as framegrabber output 82 into the video camera 80. Cable 84 provides a pathfrom the frame grabber 70 to the video camera 80 for synchronization anda return path for video camera output 83 to the frame grabber for thevideo signal.

If the pulse received by the AND gate 89a is enabled because flip flopoutput 85 is high, the strobe will pass through AND gate 89a, enter thestrobe input 94a and fire the strobe lamp 93a. The strobe lamp willproduce a very short light pulse of approximately 10 microsecondslength. The light pulse from the strobe lamp 93a illuminates the wingarea. The reflected light from any ice free specular area of the wingwill be left hand circularly polarized because of the right handcircular polarizer 30a at the output of strobe lamp 93a. Because thevideo camera 80 has a right hand circular polarizer 40 at its input, itacts as part of an isolator. That is, any reflection from a clean metalspecular area of the wing will reflect left hand polarized light whichwill not be able to get through the right hand circular polarizer 40 ofthe camera 80 and thus these areas as viewed by the camera will be verydark. The image sent by the video camera to the frame grabber will alsoappear very dark as well as the stored image that is sent from the framegrabber buffer memory into the video monitor input 79 via wire 71.

Where there is ice present on the wing it will spoil the circularpolarization of the polarized incident light and the image scene of thereflective light picked up by camera 80 and viewed on monitor 72 willnot be dimmed.

When the strobe signal passes through AND gate 89a, it simultaneouslyresets flip flop 85 to the opposite state such that AND gate 89b isenabled. Therefore, the next pulse from the pulse generator 75 into theframe grabber 70 causes the corresponding strobe pulse to be generatedwhich will be gated through AND gate 89b to energize strobe lamp 93bwhose light output is left hand polarized. Energy from strobe lamp 93bthat strikes the wing 15 and returns from clean metal will be sent intothe right hand polarizer 40 of the video camera 80. However, in thiscase, because the polarizations are of opposite hand, the reflectedlight energy that enters from specular reflecting portions of the wing15 will pass through right hand polarizer 40 and into video camera 80via lens 81 with only minor attenuation. That is, light from the lefthanded circularly polarized source 93b, 30b is changed to right handedcircular polarization upon specular reflection from wing 15 and thislight may pass freely through the video camera's right hand circularpolarizer 40.

The corresponding analog signal from the video camera that is sent tothe frame grabber 70 is recorded in its frame memory buffer and isoutput along line 71 to the video monitor 72. This particular signalwill create an image that has little difference in light intensitybetween a specular area or an ice covered area. polarization in thiscase is not important since the specularly reflected left handedcircularly polarized light will pass through the video camera's righthanded polarized filter 40. Thus, specular reflected returns and alsothe returns that come from paint or ice covered surfaces will passequally well. Accordingly, the blinking effect will be produced for thearea of a metallic surface which does not have ice on it.

Video camera 80 is preferably of the type with a built-in electronicshutter such as the Hitachi KP-M1. Because the camera shutter can be setfor a very brief time interval that corresponds to the time interval ofthe strobe lamp illumination, the camera will be especially sensitive tothe bright light from the strobe lamps and very insensitive tobackground light which will not be at a peak during the brief openshutter interval and will ignore all background light outside of theinterval that the shutter is open.

FIG. 4B shows an indirectly viewed video based ice detecting systememploying one strobed laser spotlight that makes it suitable for usewith high background illumination levels. In FIG. 4B a strobe lamp 103ais a pulsed laser which typically has an output at a wavelength in theregion of about 800 nanometers. Light from laser strobe lamp 103a issent through a right hand circular polarizer 30 and covers the field ofview in fan 22a. The light from a laser is often naturally linearlypolarized without using a linear polarizer and in such a case it may becircularly polarized by just incorporating a properly oriented quarterwave retarder plate in right hand circular polarizer 30. The right handpolarizer 30 if it includes a linear polarizer, must be rotated to theproper position so that its self-contained linear polarizer is in linewith the polarization of the laser lamp output in the case that thelaser light is naturally linearly polarized.

Video camera 80 views the scene via a narrow band interference filter104 which is centered about the strobe lamp 103a output wavelength.Generally, such a filter will have a bandpass of approximately 10nanometers and reject all light outside of the bandpass wavelength.

Reflected polarized light from the specular reflecting part of wing 15entering the video camera 80 also passes through a rotating right handcircular polarizer 140 placed in front of the video camera lens 81. Therotating right hand polarizer 140 is driven by a motor 151. A signalfrom an encoder 153 attached to the motor 151 is sent only when therotating right hand circular polarizer 140 has its plane parallel to thelens 81 at the video camera input so that the optical axes of such lensand of the polarizer are in alignment.

The analog video signal from the video camera is sent to frame grabberinput 83 via cable 84 and on the same cable the frame grabbersynchronizing outputs are sent to the video camera in-put 82. A monitorplus VCR (optional) 72 is connected to the frame grabber video outputvia cable 71. The image of the wing 15a and the image of ice area 16a onmonitor 72 correspond to wing 15 and ice area 16 which are in the fieldof view of both the fan 22a from laser strobe lamp 103a and camera fieldof view 23.

In the operation of polarizer 140, synchronous motor 151 rotates theright hand circular polarizer 140 in front of the video camera 80 at ahigh rate of approximately 600 RPM. The plane of the right handedcircular polarizer 40 lines up with the lens plane of the video cameralens 81 twice per revolution. Thus, there are 1200 times per minute thata picture may be taken. The output from encoder 153 on the rotatingshaft of polarizer 140 is used to identify each time that the rotatingpolarizer passes through such an aligned position. The two positions perrotation are alternately isolating and non-isolating and correspond tothe FIG. 1A and FIG. 1B illustrations of the isolating and non-isolatingmodes achieved by turning one of the circular polarizers.

The synchronizing pulses from the shaft encoder are sent to aprogrammable binary counter 160 which can be set to divide by anydesired number. The output pulses from the binary counter are sent tothe trigger input 77 of the frame grabber 70 along wire 76. A typicalcounter divides by any integer between 1 and 16. Counter 160 allows therate at which pictures are taken to be adjusted from a very rapid rateto a slow rate. For example, the rate at which pictures will be takenwhen the divider is set to 16 will be 1200 pictures per minute dividedby 16. To insure that alternating isolating and non-isolating states areobtained it is necessary to use only odd numbers as the divisor.

In both cases of FIGS. 4A and 4B electronic circuits are preferably usedto gate the video camera 80 to accept light only during the activeinterval of the strobe light. Also, in both cases optical bandpassfilters may be used in front of the camera to match the strobe lamp'speak wavelength while simultaneously blocking out most of thewavelengths associated with ambient lighting. The typical strobe light,a xenon flash tube, produces a 10 microsecond flash which may besynchronized to the 1/10,000 second shutter of a commercial CCD videocamera. Since the unshuttered camera would normally integrate ambientlight for at least one field, or 16 milliseconds, there is animprovement factor of 160:1, i.e., the effect of ambient sunlight can bereduced by 160:1. This 160:1 factor can be further improved by matchingthe strobe lamp (or pulse laser source) with a filter that cuts down theambient wide band light by a much greater amount than the illuminationsource.

In both the systems of FIGS. 4A and 4B the video from the video camerais captured in a frame grabber and displayed on a video monitor. Thus,if the system alternates between the isolator and non-isolator state ata 2.22 Hz rate (division=9) the picture on the monitor will be updatedevery 0.45 second and the human observer watching the monitor will seethe ice free metallic surfaces blink between dark and bright at the 2.22Hz rate.

The embodiment of FIGS. 4A and 4B effectively add an image processingcomputer which performs arithmetic operations on individual pixels inmultiple frame stores with one frame store per captured picture. Theability to perform operations on pixels allows working with portions ofthe image that are of low intensity and also provides further means foreliminating the deleterious effects of undesirable backgroundillumination such as sunlight.

Even if a curved aircraft surface region is illuminated by multipleillumination sources of circularly polarized light, it will be foundthat due to the varied orientations of the surface normal with respectto the illumination sources and receiver there will be bright regionsand dim regions in the image of the aircraft surface. The bright regionswill correspond to those areas where the surface normal has the properorientation to directly reflect the light from at least one of theillumination sources into the camera lens. The dim regions correspond tothose areas of the aircraft surface where the surface normal is suchthat the light from the illuminators is reflected predominantly awayfrom the camera lens. As previously described, portions of the imagethat correspond to an ice free surface and are brightly lit will tend tovary between white and black in successive pictures on the monitors ofthe FIG. 4A and 4B apparatus. However, portions of the aircraft surfacethat are ice free but in a dim region will vary between very dark grayand black in successive pictures and so may be difficult to identify.This problem will exist both because of the limited dynamic range of themonitor and camera and because the ratio of dark to light isintrinsically less for off-axis returns. Any remaining background due tosunlight further reduces the apparent brightness ratio between ice freeregions of successive images, particularly in the dim regions, by addingunwanted illumination to the images taken in both the isolating andnon-isolating mode.

An optimum use of the equipment of FIG. 4A and 4B is to first capture animage in the frame store that corresponds to strobing the illuminatorbut blocking the light from specular reflection from ice free metal;e.g., capture a picture in the isolator mode. Next, the illuminator isstrobed and a picture is captured in the non-isolating mode. Finally, apicture is captured with the illuminator strobe off, capturing a picturethat consists purely of the undesirable background light. If thereceiver (detector) optics is being varied between pictures to changebetween the isolating and non-isolating mode of operation, it is notimportant which mode it is in when the background image is capturedbecause both modes will have been balanced for equal light attenuationof unpolarized light.

The digital value corresponding to the background illumination in eachpixel of the frame grabber holding the background may now be subtractedfrom each corresponding value in each of the pixels of the image inwhich specular returns were blocked; i.e., from each pixel of the imagetaken in the isolating mode. The process of subtracting the backgroundis repeated for each pixel in the frame grabber holding the image takenin the non-isolating mode. At this point, assuming linearity of thepixel values, the effect of any remaining background light has beenremoved from the two frame stores. If the recording or digitizingprocess is not completely linear the non-linearity must be removedbefore performing the subtraction. This is normally performed at thetime the image is first digitized and entered into the frame store viathe use of a look up table in the image processor and is well known inthe state of the art.

Once the images in the frame grabbers have had the effects of backgroundillumination removed, the image processor can find the ratio ofamplitudes between corresponding pixels in the two images. By forming aratio of the value of the intensity of the pixels in the second(non-isolating) image divided by the value of the intensity of thecorresponding pixels in the first (isolating) image, a ratio havingvalues generally equal to one or greater than one will be obtained. Icefree metallic surfaces that have surface normals reflecting theillumination towards the camera lens will have the highest ratios. Anormalizing value approximately equal to one divided by the Nth root ofone divided by the largest of the two pixel values that created theratio (generally, the value of the pixel from the non-isolating picture)may be used as a multiplier to enhance the ratio from the ice freesurfaces that are dim due to their being off-axis with respect todirecting the reflected light towards the camera. N is typically aninteger equal to or greater than 2. Of course, only values higher thansome chosen threshold should be so normalized so that the system doesnot respond to noisy signals. If desired, the preceding arithmeticmanipulation of pixel values may instead be performed on groups ofpixels that correspond to segmented or filtered portions of the aircraftsurface image. These filtering techniques which include low pass spatialfiltering and median filtering may be used to operate on noisy imagesand are well known in the state of the art. Another suitable metric forcomparing corresponding isolating and non-isolating pixel or regionbrightness amplitudes is the normalized difference. This may be formedby subtracting corresponding pixel or region amplitudes and dividing theresult by the sum of their amplitudes.

To highlight ice free regions in the most easily interpreted form, theratios may be assigned to colors as, for example, that high ratioregions are assigned to the color green, low ratio regions to the colorred, and intermediate ratio regions to the color yellow. These colorsmay be used to color the non-isolator image on the screen of the colorvideo monitor. Optionally, the ratios may be encoded in black to whiteintensity levels that may be displayed in the same manner as the colorencoded images. Such levels may be used to indicate ice thicknessaccording to the amount of depolarization observed.

All of the preceding techniques of using isolator and non-isolatorstructures may be implemented by using linearly polarized light in theilluminator, rather than circularly polarized light, and equipping thereceiver (detector) with a linear polarizer that is alternately alignedwith and then at right angles to the polarizer in the illuminator. Thismode of operation depends upon the fact that an ice-free metallicsurface will return polarized light approximately unchanged whereas anice covered metal surface or matte material will de-polarize the light.Thus, once again, an ice covered metallic surface will remain atapproximately the same intensity. Of course, the transmitted linearpolarization can be alternated between being aligned with and then beingat right angles to the direction of a linear polarizer in the receiverto achieve the same end.

FIG. 5A shows the details of the FIG. 4B rotating circular polarizer 140and video camera 80 assembly. Video camera 80 is mounted to a bracket150. A motor 151 is also mounted to bracket 150 and has a slotted outputshaft 152 for holding the circular polarizer 140 to rotate insynchronism with the shaft. An encoder disk 153 mounted on shaft 152 isused to sense the position of the rotating polarizer 140. Encoder disk153 has a photo optical interrupter 154 supported by a member 157affixed to bracket 150. The encoder disk is solid everywhere except fortwo positions, 180° opposite each other, which are in line with photointerrupter 154 only when the optical plane of polarizer 140 is parallelto that of the lenses in video camera lens assembly 81.

A top view of this arrangement is shown in FIG. 5B and FIG. 5C whichshows an encoder pickup 154 which incorporates an LED light source and aphoto diode in one package 155 that is commercially available as Optekpart number OPB120A6.

FIG. 6 is an optical schematic of the laser diode spotlight assembly.Light from laser diode 300 is collected by a collimating lens 301 andthe collimated beam is sent into the telescope formed by a negative lens302 and a positive lens 303. When the focal point of the positive lensis coincident with that of the negative lens, a collimated beam emergesfrom the positive lens 303. Positive lens 303 is shown in position 310so that its focal point coincides at 312 with that of negative lens 302.A collimated beam 304 is the result of this configuration. When thepositive lens 303 is moved closer to the negative lens, such as toposition 311 in FIG. 6, the beam 306 that emerges is expanding and socan cover a wider field of view. Thus, by adjusting the position of thelens from 310 to a point where it is close to the negative lens, it ispossible to obtain any output light cone between collimation and a coneslightly narrower in angle than that of the beam 313 as it leavesnegative lens 302. The arrangement of FIG. 6 is also applicable to allother illumination sources shown when the source (filament or flashlamp) is small.

FIG. 7 shows an indirect viewing system for ice on metal detection thatuses a synchronous detection method that can operate with onephotosensor or an array of photosensors, according to the field of viewand resolution required. The illumination source for the surface area 15to be inspected is not shown but it may be any bright source of eitherright handed or left handed circularly polarized light.

The area that is to be inspected is imaged via camera lens 400 ontophotodiode 402, or onto an array of similar photo diodes. The circularpolarizer required for isolation is formed by quarter wave retarderplate 442, linear polarizer 441 and Verdet rotator 401. The Verdetrotator is typically of garnet and energized by a magnetic field createdby a coil 450 via power buffer amplifiers 402a and 402b whichalternately drives current through the solenoid, first in one directionand then in the other. The effect is to cause linearly polarized lightfrom the quarter wave plate passing through the rotator to change thedirection of its polarization by plus or minus 45° according to thedirection of the solenoid current flow. Other devices based on the Halland/or Pockel's effect which use high voltage fields could be used in asimilar manner.

In the static condition with no current flow through the coil 450, bothright handed and left handed reflected circularly polarized light fromwing area 15 will pass through the polarizer 441 to the photo diode 402with little attenuation because the slow axis of quarter wave plate 442is in line with the polarization axis of linear polarizer 441.Therefore, light of either hand circular polarization is at 45° to thepolarizer and so can pass through polarizer 441 without largeattenuation. However, when the coil is alternately energized withcurrent flow in opposite directions, the addition and subtraction of 45°to the plane of polarization present at the output of the quarter waveplate 442 causes the plane of polarization to alternate between verticaland horizontal at linear polarizer 441. Thus, reflected circularlypolarized input light will alternately be allowed to pass and not passto the photo diode detector.

Because the rotation of the plane of polarization is performed viacurrent direction switching, it can be performed quite rapidly. A 10 KHzrate, which is adequate for the apparatus, is easily obtained. A clocksource 406 provides pulses to a flip flop 403 at its toggle input 405.The flip flop 403 outputs 404a and 404b are amplified by buffers 402aand 402b to energize coil 450 in a direction that varies according tothe state of the flip flop 402.

The optical energy received at the photo diode array 402 generates acorresponding electrical signal that is applied over input line 408 to adifferential amplifier 407. The output 409 of amplifier 407 feeds twobuffer amplifiers 410a and 410b via their inputs 411a and 411b. Bothamplifiers 410a and 410b have equal gain but are of opposite polarity.

A multiplexer 413 has its inputs 414a and 414b connected to the twoamplifier 410a and 410b outputs 412a and 412b. The multiplexer 413directs its two inputs to its single output 415 according to the stateof its select terminal 460 which is connected to output 404a of the flipflop 403. The output 415 of the multiplexer 413 is applied to anintegrator 416 or optional low pass filter 426. The integrator 416 isformed by input resistor 420, operational amplifier 417, capacitor 421and field effect transistor 422 which is used to periodically reset theintegrator by discharging the capacitor. This arrangement is well knownin the art. The integrator 416 (or filter 426) output 419, when greaterthan a threshold voltage positive or negative as set by a double endzener diode 480 and current source resistors 425a and 425b, willenergize one of the oppositely poled LED's 424a or 424b.

The detection circuit of FIG. 7 rejects the light reflected from diffuseor ice covered areas but passes that from ice-free specular surfaces.Diffuse or ice covered surfaces return unpolarized light to thedetector. With these type surfaces, although the current direction inthe Verdet rotator 401 is changing direction at a 10 KHz rate, the lightreceived by the photo diode 402 remains at a constant level, i.e., thelight amplitude is unchanged because the light is not polarized.

The electrical voltage at the output 409 of amplifier 407 responds tothe input level and remains constant. The multiplexer 413 alternatelyselects equal constant level positive and negative voltages so that theintegrator 416 (or low pass filter 426) output 419 stays close to zeroand neither of the LED's 424a or 424b draw current since the outputvoltage does not overcome the zener diode 480 threshold voltage.

It can be seen that when area 15 is ice free the light returned to theapparatus will be circularly polarized and the signal at photo diode 402will alternate between a large and small value at a 10 KHz rate. Sincethe two voltages selected at terminals 414a and 414b will differ inamplitude, they will not average to zero at the output of the integrator416 (or low pass filter 426) and one of the LED's will light, accordingto whether the larger of the two voltages at point 409 was received whenthe state of flip flop 403's output 404a was high or low. This, in turnusually depends upon whether a right handed or left handed illuminatoris being used. The output LED can also change if the wing area 15 beingobserved receives most of its circularly polarized illuminationindirectly via specular reflection from another surface, since each suchreflection changes the state (hand) of the circular polarization.

The apparatus of FIG. 7, when used with a single photo-detector, isuseful with a mechanical drive apparatus that scans the optical axis ofthe assembly in both elevation and azimuth to generate a raster scanwhich will create a full image of a scene on a point by point basis. Theoutput 419 may be sent to a video display which is being scanned via itsdeflection circuits in synchronism with the mechanical drive apparatusto paint the image on the screen. As an alternative, the optical axismay be scanned in a raster pattern using azimuth and elevationdeflecting galvanometer arrangements such as are available from GeneralScanning Corporation. Of course, such synthetically generated images mayalso be digitized and processed using the image processing hardware andsoftware techniques previously described.

FIG. 8A shows an embodiment of the invention useful when it is importantto obtain ice detection information in an extremely rapid mode that isuseful for scanning across an object in a short time without thesmearing or the misregistration that may occur when the camera ispanning and sequential pictures are taken for the isolating andnon-isolating modes.

In FIG. 8A, a strobe lamp 103a is used with linear polarizer 501 toilluminate the wing surface 15 via polarized light in fan 22a. The videocamera 80 with lens 81 images the scene as contained in field of view 23which overlaps fan 22a. A polarization preserving beam splitter 503 isused to divide the energy received by lens 81 into two substantiallyequal amounts which are directed to video cameras 80 and 80a. Camera 80is fitted with linear polarizer 500 which is in alignment with linearpolarizer 501 so that reflected specular energy may pass with littleloss and so creates a non-isolating mode receiver Camera 80a also isfitted with a linear polarizer 500a but its axis is aligned at 90degrees to that of linear polarizer 501 so that reflected specularenergy is blocked which creates an isolating mode receiver.

When the synchronizing pulse is received via wire 87, the strobe lamp103a flashes for a brief time; 10 microseconds is typical. During thebrief flash interval the isolating and non-isolating images are capturedon the silicon CCD devices (typical) in the two cameras, 80a and 80,respectively. The two images can be read out sequentially via amultiplexer and recorded in the digital frame buffers of the imageprocessor. A multiplexer of the type required is built into the Cognex4400 and is normally part of most commercial frame grabbers and imageprocessors. The processing of the images is substantially the same aspreviously described with amplitude comparisons being made betweencorresponding pixels or corresponding regions.

Because the two cameras use a common lens 81 the images will have topand bottom reversed (one is viewed through a mirror) but are otherwisesubstantially geometrically identical. Calibration may be obtained byrecording any two points in the field of view and mechanically adjustingthe CCD chips via translation and rotation to have a one to onecorrespondence of pixels. This can also be accomplished via softwarewithin the image processor and such conventional software as is normallyfurnished with the image processor. Because the lens 81 and cameras 80and 80a are held in alignment, the calibration, whether via mechanicalor software means, need only be performed once, at the factory.

In FIG. 8A, the linear polarizers may be replaced with circularpolarizers such that at least one of the circular polarizers in thereceiver has the same "hand" as that of the transmitter to provide anisolating mode image and the other has the opposite "hand" or not becircularly polarizing and have suitable attenuation to ensure thatdiffuse objects have the same intensity in both pictures. Additionally,if polarizing beam splitters are used, one or more of the polarizers inthe receivers may be omitted since polarizing beam splitters will divideenergy according to polarization properties.

In FIG. 8A, the isolating and non-isolating images may be obtained withtwo separate cameras as shown, but with two separate and substantiallymatched (in focal length and axis parallelism) lens means, one percamera, that create geometrically corresponding images. Thecorrespondence need not be exact if corresponding image features orregions or pixel groups are compared with respect to average amplitudein the isolating and non-isolating mode.

An alternative arrangement shown in FIG. 8B, section view, uses a mirror510 in each path and so does not invert one image with respect to theother.

As can be appreciated, the camera in all embodiments may be replacedwith a multiplicity of cameras at various positions and angles to theilluminated surface to gather more of the specularly reflected light andsimilarly, a multiplicity of illuminators may be used at variouspositions and angles to the illuminated surface to assist the cameras ingathering more of the specularly reflected light. It is only necessarythat when such arrangements are used that all control signals andpolarizers be common to the group of cameras that replaces one camera orto the group of illuminators that replaces one illuminator.

The arrangements of FIG. 8A and FIG. 8B require multiple cameras andbeam splitters which are similar to first generation color cameras whichemployed three separate cameras to separately record three separateimages, one for each of the primary colors. More modern color camerasemploy a single camera with a patterned color filter that is organizedin closely spaced columns; e.g., R,G,B,R,G,B,R,G,B . . . where Rrepresents red, G represents green and B represents blue. This has theadvantage of using only one camera plus simple electronics and requiresa one time adjustment of the filter to the camera chip at the factory.The same identical color camera pickup chip and electronics circuits maybe used to manufacture a polarization sensitive camera by replacing thetricolor filter used in the color camera with the two layer filter shownin the assembly of FIG. 9A.

In FIG. 9A, the camera pickup is represented by CCD chip 900 withtypical scan lines 901. A thin linear polarizer 910 with polarizationaxis at 45 degrees to the "slow" axis defined for patterned retarderplate 920 is located touching, or in close proximity to the illuminatedsurface of the CCD chip. Retarder plate 920 is manufactured from abirefringent material and selectively etched so that adjacent columnsdiffer by 1/4 wave with respect to the retardation produced and apattern of +,0,-,+,0,-,+,0,-, . . . is maintained where "+" represents+1/4 wave (923), "0" represents equality of phase (922), and "-"represents -1/4 wave (921). The patterned retarder plate must be inclose proximity to the polarizing plate. The retarder plate selectiveetching may be done chemically or with ion beams and is well known inthe semiconductor industry. The process is currently being used tocreate micro lens arrays known as binary optics.

The arrangement shown in FIG. 9B requires two mask and etch steps toobtain the three thicknesses needed for manufacturing the threeretardations needed for the three column types. The optional filling 924adds non birefringent material having an optical index approximatelyequal to that of the birefringent material to provide the overallstructure of a thin glass plate with respect to a focused light beam. Asshown in FIG. 9B, the columns are brought into alignment with the pixels905 in a CCD column in exactly the same manner as is done for a colorcamera.

In operation, the polarization images produced by the patterned retarderplate will be processed by the color camera's electronic circuits intoeither three separate images or a single composite image. In the casethat a single composite image results, it can be decoded by any colorreceiver into corresponding R,G,B images which will represent not thethree colors but the three states of circular polarization receivedwhich correspond to left, right and non polarized. These images may beprocessed according to all of the preceding methods regarding icedetection.

Although all cameras shown have been of rectangular format, in somecircumstances it may be preferable that a linear camera array (singlerow of pixels) be used and the field of view be transversely scanned viarotating polygon mirrors, galvanometers, rotating prisms, or otherscanning means to synthesize a rectangular image of some desired format.At such times the illuminator may provide a "line of light" which wouldbe likewise scanned in synchronism with the scanning of the lineararray. This is suitable for fields of view which may be long and narrowand require more resolution than may be obtained from the standardcamera format.

FIG. 13 shows a scanning ice detection system. A laser diode lightsource 13, such as Spectra Diode Laboratories SDL 5422-H1 capable ofproducing short, bright pulses of light, is projected onto path foldingmirror 18a which reflects the light pulses through polarizing prism 603a(a linear polarizing plate could be used when using just one lightsource 13). Prism 603a linearly polarizes the light, reflecting theunwanted orthogonal polarized light to be absorbed by absorber 17. Thelinearly polarized light passes through quarter wave retarder plate 12a,converting the light to circular polarization. Mirror 18c mounted onlens 81 folds the light path to coincide with the receiver light path.

A galvanometric scanner 217, such as Laser Scanning Products GRS-PS,series scans the light over an angle 219 in the horizontal plane viaoscillating mirror 218. Scanner 217 is rotated in the vertical plane bymotor 63 driving shaft 64, causing the horizontally scanned light toalso scan vertically. Light reflected by a surface illuminated by thescanned light retraces the path to a positive lens 81 which focuses thelight in combination with a negative lens 281 onto pinhole 282 inbarrier 283. Light passing through pinhole 282 is focused by a lens 284upon avalanche photodiodes 180 (such as the 5 mm avalanche photo-diodesfound in the Advanced Photonix APM-10 Detector modules) after passingthrough quarter wave retarder plate 12b and polarizing prism 503. Mirror18b folds the light path. Plate 12b converts the circularly polarizedlight to linearly polarized light, which if aligned with thepolarization axis of prism 503 passes through to mirror 18b and onediode 180. If the polarized light is orthogonal to the polarization axisof prism 503, it will be reflected to the other diode 180 by prism 503.Unpolarized light will illuminate the photodiodes 180 equally whereasproper alignment of the circularly polarized light from light source 13,prism 503 and plate 12b can produce a maximum difference in light levelson one diode 180 relative to the other diode 180 when light projected bythe system is specularly reflected. Narrow band interference filter 104,centered about the wavelength of light source 13, reduces the amount ofambient light which reaches photodiodes 180.

Quarter wave retarder plates 12a and 12b may be removed from the systemand the system will then use linearly polarized light to produce a largeratio difference in light levels on photodiodes 180 for specularreflections and equal light levels on photodiodes 180 for unpolarizedreflections.

An alternative system adds a second light source 13 which passes lightaligned to the polarization axis of prism 603a onto absorber 17 andprojects light orthogonal to the polarization axis of prism 603a whichis reflected by prism 603a through plate 12a along the same path as theother light source 13 by careful alignment. The polarizations of thelight from the two sources 13 are of opposite hands so that only onediode 180 is required to detect the large ratio of light reaching thediode from the two sources when reflected by a specular surface. Theunpolarized light reflected from either source 13 will reach a diode 180with equal intensity if the source 13 levels are equal. Thegalvanometer's mirror will typically scan 3 meters in 1/400 sec orequivalently travel 0.012 cm in the time between strobing the two lightsources 13/100 nsec apart. Thus the diode receives light reflected fromessentially the same area from the two sources. As previously noted,plates 12a and 12b may be removed.

When one light source 13 and two photodiodes 180 are used, the diodereceiving the large specular reflected light may tend to heat up andalter any calibration of signal level. Using two diodes and two lightsources 13 can reduce this problem. By alternately strobing the lightsources 13, the heating will be equal since the large specular energywill alternate between photodiodes 180.

Although the equipment described separates clear wing from ice and snow,it does not separate (except visually to the operator's eye) runway andother background surfaces from wing surfaces, etc. This can be done viaimage processing techniques or stereo ranging or lidar (optical radar)ranging. Also, image processing techniques to be employed can segmentsurfaces of like texture and only color red those "non-blinking" areasthat are substantially surrounded by "blinking" areas (green). That is,ice would be highlighted only when substantially surrounded by clearmetal. As an alternative, stereo ranging may be used to separateforeground from background and only the foreground (wing or otheraircraft surface) have non-blinking areas tagged to highlight iceformation.

When viewing downward on an aircraft wing, the ground appears in thefield of view and returns unpolarized light similar to ice. One methodof rejecting this unwanted signal is to use the time of travel of thelight pulses to determine which surface is reflecting the light. If thewing is at least 5 feet above the ground, the ground signal will reachthe diode 180 at least 10 nsec later (curve 3, FIG. 14) than a signalfrom the wing (curve 2) relative to the time of the strobe 289 (curve1). Thus any signal exhibiting this additional delay can be rejected asnot belonging to the wing. Since the measurement can be made on theleading edge by measurement unit 286, pulses wider than 10 nsec can beused without effect on this rejection process.

The problem of large dynamic range of reflected light from specularsurfaces is not as great for the scanning system since the dynamic rangeof avalanche photo diodes (such as used for the APM-10 detectors) ismuch larger than that which is available for most imaging cameras. Todeal with the large dynamic range of the output of these diodes, it ispreferable to use a logarithmic amplifier 280 such as model AD640 fromAnalog Devices. Since ratios of signal levels are being analyzed, thelog amplifier has the additional benefit of producing the same voltagechange for a fixed ratio throughout the dynamic range.

A further problem, common to both scanning and non-scanning systems, isthat the ratio of measured values in the isolator and non-isolatorstates reduces towards that of ice as the viewing angle deviatessignificantly from normal incidence when viewing a specular surface.This increases the difficulty in processing signals over a wide dynamicrange.

To further overcome the influence of wide dynamic range on the displayproduced by the system, the display of the images can be enhanced byprocessing via computer 287 the signal level produced by each pixel inthe image of camera 80 or by photodiodes 180 and to quantize the displayfor that pixel (or scan point for the scanning system) into threelevels: clear, ice and non-ice. A background reference level is obtainedfor the pixel (or scan point) when the light source 13 is not beingstrobed. This will account for any ambient light. The short strobe timepossible with the scanning system essentially eliminates interference byambient light. The measurements are then made for the pixel (or scanpoint) by strobing twice; once with the optical means in the light pathbetween light source 13 and camera 80 (or diode(s) 180) in an opticalisolator state and once in an optical non-isolator state. Apredetermined table of threshold values as a function of the measuredbackground reference level and the value measured in the non-isolatorstate is stored in computer 287. A further problem that can be addressedby the table is the reduced ratio of measured values obtained in theisolator and non-isolator states when viewing a specular surfacesignificantly away from the surface normal. If the ratio of thenon-isolator state value to the value measured in the isolator stateexceeds the threshold value from the table, the pixel is declared to bein a clear area, otherwise it is declared to be in an ice area. If theisolator state measured value in the ice area is less than a givenvalue, the pixel is declared to be in a non-ice area. By displaying ondisplay 72 the three categories as black, white and grey, or variouscontrasting colors, the display readily conveys the desired informationconcerning the icing condition on wing surface 15.

When multiple levels of illumination are used to increase the dynamicrange of the system, the above method for quantizing the measurementsinto three levels will tend to declare "ice" for one or more of thelevels when the declaration should be "clear". Thus if a declaration of"clear" is made at any level of illumination, the pixel is declared in afirst decision to be in a clear area. The measured value in the isolatorstate with the highest illumination is used in a second decision todetermine "non-ice" in areas not declared "clear" by the first decision.If time of arrival is used to eliminate ground returns, then thosesignals of a continuous group that are received significantly later thansignals of another continuous group are declared "non-ice". A processflow diagram is provided in FIGS. 16A-16C for the steps described above.

FIG. 13 provides the details of the scanning system signal flowsindicated above. Timing and control unit 285 synchronizes strobes 289 tothe oscillating mirror 218 (synchronizing signal not shown). Strobes 289cause light sources 13 to emit light pulses. Logarithmic amplifiers 280receive diode 180 output signals 293 and send their compressed dynamicrange outputs 290 to analog to digital converter 288 and leading edgemeasurement unit 286. Leading edge measurement unit 286 starts measuringtime at the time of strobe 289 and stops when input signal 290 exceeds agiven value. The time interval thus measured is transmitted to computer287 via signal 294. Analog to digital converter 288 reports theamplitude measured on input signal 290 to computer 287 via signal 291.Computer 287 quantizes input signal values 291 into categories of clear,ice and non-ice as described above, using leading edge time on signal294 to force background signals to be classified as non-ice. Thecategories are transmitted to display 72 via signal 292 from computer287 to provide a visual image of wing 15 and ice patches 16.

Leading edge measurement unit 286 can use any standard processing meansthat provides adequate time resolution. FIG. 17a illustrates thewaveforms of an analog embodiment. Strobe 289 from timing and controlunit 285 is shown as pulse waveform 171 which establishes time=0 formeasuring time of travel for light to reach surface 15 and return tophotodiodes 180. A sweep voltage having waveform 172 is started at theleading edge of pulse 171. The waveform 173 of the output 290 ofamplifier 280 is compared to a threshold and triggers a track and holdcircuit to sample the sweep voltage (or alternatively just stops thesweep) providing waveform 174. Waveform 174 is then converted to adigital value at time A by an A/D converter, where time A exceeds themaximum expected signal delay time.

FIG. 17b illustrates an embodiment using a digital counter. Again,strobe 289 is shown as waveform 171 to establish time=0. Waveform 175indicates the waveform of gated clock pulses that are turned on by theleading edge of pulse 171 and turned off after the maximum expectedsignal delay time. When amplifier 280 output 290 shown as waveform 173exceeds a threshold, a counter counting the clock pulses is stopped andholds its count as indicated by waveform 176 (alternatively the clockpulses can be stopped when waveform 173 exceeds a threshold). At time Athe counter value is sampled for use by computer 287.

The computer 287 can further improve the display by comparing thedeclared category of a pixel over several scans. If an indication of"clear" and "ice" alternates, then it can be concluded that ice has notformed and the sporadic declarations of "ice" are caused by blowing snowwhich can be displayed as "clear" or a fourth category.

As indicated above, a problem exists when viewing a wing surface at anangle significantly away from the surface normal. The ratio of thevalues measured in the isolator and non-isolator states approach that ofice, thus preventing reliable discrimination. Certain paints, materialsand surface treatments can be applied to remove this deficiency.

Currently some aircraft wings are painted with a gray paint thatprovides a low ratio of isolator to non-isolator response. Adding 10% byvolume of metal chips to the paint significantly improves the responsebut not as much as is desired. However, it has been found that asignificant response improvement can be obtained by using black paint towhich 10% by volume of metallic chips have been added. Surprisingly, anobserver sees the appearance as similar to the gray paint, probablybecause the specular reflection of the metallic chips produces a grayappearance.

FIG. 15 shows the ratio of response obtained by the ice detection systemwhen viewing various surfaces as a function of the angle the systemmakes to the surface normal. The bare metal curve 1 starts at a ratio of66 on the surface normal and drops to 3 at 12 degrees from normal.Measurements of paper (curve 3) which is similar to that received fromice starts at 1.55 on the surface normal and drops to 1.47 at 12 degreesfrom normal. Thus at least a 2:1 difference in ratios exists out to 12degrees from normal, and reliable discrimination is possible. Beyondthat, performance becomes gradually less reliable.

Curve 2 is for a paint with approximately 10% metal chips added byvolume which, when applied to the surface, provides a ratio of 3 out to30 degrees from normal. Other currently existing aircraft paints providea similar response.

Retroreflector tapes and paints that contain reflecting sites show verylittle sensitivity to the angle of view. Curve 4 shows that a surfacecovered by 3M silver provides a ratio greater than 27 out to 50 degreesfrom normal. 3M Reflective Highway Marking Tape series 380 (white) andseries 381 (yellow) are used on roadways as reflectors and therebyprovide the basis for detection of ice formation on roadways andbridges. Many of the suitable retroreflective tapes are manufactured byembedding tiny metallic coated dielectric beads in a clear plasticcarrier. The tiny spherical beads reflect a portion of the impinginglight back towards the light source, substantially independent of itsdirection. In fact, the reflected energy is generally much larger thanrequired and can saturate the receiver. It is therefore preferable toadd an attenuating layer of material to the surface of theretroreflective tape to reduce its response. An attenuation ofapproximately 3:1 each way is recommended as a best value (approximately10:1 roundtrip loss).

Another method of reducing the sensitivity of response to angle fromsurface normal from a metallic surface is to cause the surface topresent many small facets at all angles so that at any viewing angle asignificant number of facets will have their surface normal essentiallyaligned to the viewing angle. Sand blasting, roll dimpling, etching andother methods are in common use to produce surfaces with thischaracteristic.

The invention is to facilitate ease of use and promote record keepingwhich is of vital importance to the aviation safety industry. Althoughnot shown in the various drawings, it is anticipated that flight number,aircraft identification, time and date and other pertinent informationwould be aurally, visually, or textually annotated to the displaymonitors and to the disk or tape recordings made with the ice detectionequipment. The performance of this task would be implemented withcommercially available components that are often part of the equipmentspecified (cameras and recorders) or via additional "plug compatible"annotation and editing devices.

It may be desirable to locate the recording and viewing or controlequipments at remote locations such as the aircraft cabin, controltower, ground control area, or aircraft terminal. Cameras andilluminators may also be built into various remote portions of theaircraft from which the wing or other surface is to be monitored.Accordingly, the various wires shown in the drawings, whether forpurposes of data or signal transfer or control, may be replaced withtelemetry equipment operating via radio, infra-red, power lines or fiberoptic links.

In all claims and in the foregoing disclosure the term "light" is to beinterpreted as "electromagnetic energy" and not restricted to just thevisible light portion of the electromagnetic spectrum inasmuch as theprinciples described are not so limited and in fact extend into theinfrared and beyond.

The precise ice-present/ice-free decision threshold for the ratio formedby the non-isolating state received signal amplitude divided by theisolating state received signal amplitude is a function of the anglebetween the normal to the viewed surface and the FIG. 13 equipment lineof sight. The ice-free non-isolating to isolating ratio is a function ofboth angle and material as can be seen in FIG. 15 since the signal ratioof an ice-free surface point or area correlates strongly with these twovariables. When the FIG. 13 or other similar equipment is used in thefield, frequently the angle between the equipment optical path ormaterial being viewed is unknown. Accordingly, it is useful to obtain asingle threshold function for ice-detection that applies across a widerange of materials and angles so that precise knowledge of optical pathangle or surface material is not required.

A one-time calibration can be made for a set of materials to obtain afunction in which the threshold is primarily a function of the amplitudeof the received light reflected from the surface or the material thereonwith the optical element in the path in the non-isolator state. This isdone by generating a curve for each material similar to that of FIG. 15,both for ice-free and ice-covered surfaces. Once such a curve isgenerated for a particular material, it need not be redone. Rather, theinformation so gathered can be combined with the information gatheredfor any other particular collection of materials that may be encounteredin order to create an optimized decision boundary (threshold) for theice-present/ice-free decision. The generation of such a curve isexplained below.

When a predetermined set of materials will be encountered in the field,the information used to derive the curves of FIG. 15 can be plotted in amore useful manner by having (1) the amplitude (brightness) of thereceived reflected non-isolator optical state signal replace the angleas the abscissa and (2) the ratio, or other mapping function, of therelationship between the amplitudes of the non-isolator and isolatoroptical state received signals plotted as the ordinate. An angle is notnecessarily the best auxiliary data to use with the non-isolating andisolating received reflected signal amplitudes to determine thethreshold ratio at which the amplitudes of the received reflectedsignals are substantially equal (to indicate the presence of ice orsnow). This is because it requires determining the local surface normalof the viewed region whose data is being evaluated with respect to theline of sight of the system receiver's sensor. Obtaining the localsurface normal data requires obtaining accurate and relatively noisefree range and position data for each small area in the subject surfacethat is to be evaluated. This is not always possible.

For metallic surfaces, the non-isolator received signal amplitude itselfis indicative of the angle between the local surface normal and thereceiver sensor line of sight. It is well known that when a viewedspecular reflecting surface is normal to the line of sight of a coaxialreceiver/transmitter arrangement, such as shown in FIG. 13, the returnreflected beam will be extremely bright, i.e., a high amplitude receivedsignal, and that when the specular surface deviates significantly frombeing normal to the line of sight the return beam will be much dimmer,i.e., a lower amplitude received signal. This is because most of theenergy reflected from the surface will be directed away from thereceiving lens of the receiver. In a similar manner, when surface 15 ofFIG. 3B is in such a position and at such an angle that the raysemerging from the source illuminator are specularly reflected by theviewed surface directly into the receiving lens, the amplitude of thenon-isolating state received signal will be extremely strong.Conversely, when this optimally specular arrangement is farther frombeing satisfied the received signal energy amplitude will rapidly falloff in intensity. Thus, the deviation of the angle from the optimumnormal specular arrangement between the illuminating source and thereflecting surface and the receiving lens and the surface may be roughlyinferred from received signal amplitude information.

FIG. 18 is a plot indicative of the ratio obtained by dividing thereceived signal amplitude observed in the optical non-isolator(non-blocked) mode by the received signal amplitude obtained in theisolator (blocked) mode versus the non-isolator mode received signalamplitude for the various materials indicated. Since a large dynamicrange must be covered for both the non-isolator signal amplitude and thenon-isolating/isolating signal ratio, the FIG. 18 plot is presented in alogarithmic scale for both axes. It is also normalized with respect to areference diffusing white lambertian surface; e.g., a surface with nearunity reflectance that returns nearly equal energy in both the opticalisolating and non-isolating modes. This is, for example, white paper.This reference point is the origin 601 of the FIG. 18 graph.

The portion of the curves for each material that lie towards thepositive most portion of the abscissa, i.e., to the right of the origin,are those points that most nearly correspond to receipt of specularreflection of light normal to the receiver sensor, such as shown in FIG.13, since they correspond to the largest amplitudes of receivedreflected signals and are far larger in amplitude than the amplitudes ofreflected signals that would be obtained from a white diffuse surface.Points on the curves that lie to the left of the ordinate (left of theorigin) correspond to specular surfaces that are far from being normalto the receiver (e.g., FIG. 13) sensor axis and therefore have signalamplitude returns weaker than would be obtained from a white referencesurface. The area 606 with boundary 607, which corresponds to the regionwhere received signal amplitude from ice covered metallic surfacesoccur, is cross hatched.

A threshold curve 608 which may be used to separate ice and snow coveredsurfaces from clean metallic wing surfaces is drawn between the crosshatched area and the "clean wing curves" for bare aluminum 602, metallicpaint 603 and retroreflective material 604.

The data for the curves shown in FIG. 18 may be obtained by recordingthe isolator and non-isolator state received signal amplitude responsesobtained with the equipment of FIG. 13 or FIG. 3B (the eye beingreplaced with a photo detector) over a wide selection of angles betweenthe ice detection sensor and the surface at a fixed working distance foreach such material or surface condition that is to be investigated. Thisis most advantageously done by rotating the viewed surface about an axisthat is normal to the sensor axis and passes through the sensor axis.After the mathematical computations are made the curves are plotted fromthese data. The data are taken at constant range to ensure that thesignal amplitude obtained in the non-isolator mode is not altered by thechange in the working distance. Otherwise, appropriate corrections mustbe made for such range variation which will generally be in accordanceto the inverse square of distance.

When the ice detection equipment is used at an approximately constantworking distance from the surface being inspected the curves of FIG. 18may be used to determine the threshold between ice or snow present andice-free conditions. During investigation of a surface, at the receiversensor each pixel group or pixel, respectively, corresponds to a surfacearea or point, respectively, and is associated with an opticalnon-isolator and an isolator pair of received signal amplitude numbersas the system alternates between non-isolator and isolator states. Inthe case of a pixel group, each number of the pair of numbers willnormally be the average or otherwise filtered value representative ofthe non-isolator or isolator state received signal amplitude of thegroup of pixels. The value of the non-isolator number of the pair may beused for the entry point (corresponding to the abscissa of the FIG. 18graph) into a table (or function) that represents the threshold curve608 of FIG. 18. If, at the point of entry set by the non-isolatornumber, the ratio of the non-isolator amplitude number of the pair ofnumbers to the pair isolator number lies above the threshold 608, it isconcluded that the surface is ice free. Conversely, if the computedratio lies below the threshold value for that entry point it isconcluded that the surface is not ice free.

In actual construction of such a table or function, the origin may beset at any convenient point. Use of white paper (reflectance equal tounity) as an origin is convenient primarily for human interpretation.Such an origin puts strongly specularly reflected light to the right ofthe ordinate and weakly reflected light (specular or non-specular) tothe left. Similarly, increasing specularity is defined by light atincreasing distances above the abscissa. The computation and curveevaluations of received signals are easily carried out during actualsystem use by use of a computer which includes a look up table.

When the detection equipment is not used at constant working distancesfrom the surface being investigated, the table or function based on theFIG. 18 threshold curve 608 still may be used but it must be entered(abscissa) via an offset that takes into account the range to thesurface being viewed versus the range at which the table wasconstructed. For instance, if the table or function was constructed fromdata collected at a range of 16 feet and the distance to the surfacebeing interrogated is 32 feet (twice the distance) the received signalamplitude (for the FIG. 13 sensor) will be approximately 1/4 of that atthe 16 feet (reference) distance.

Unlike a similar calculation for radar or sonar, only the one waydistance need be taken into account since all of the area illuminated bythe transmitter source is imaged by the receiver lens onto its detectorsurface. Thus, if the object surface is at twice the reference distance,the lens of the receiver detector will subtend 1/4 the solid angle asviewed from the illuminated surface point and will collect onlyapproximately 1/4 of the reference energy. Accordingly, to enter thetable at the proper place it is necessary to perform the look-up orfunction evaluation at four times the actual amplitude of the measuredreceived signal. Therefore, if logarithms to the base 10 are used in thegraph plot, the log of 4 would be added to the log of the non-isolatorpixel group number received signal amplitude value to obtain the(abscissa) entry point because adding the log of 4 is equivalent tomultiplying by 4. The same correction applies when either taking datawith 1/4 of the transmitted power or with 1/2 of the receiver aperturediameter. The ratio of the non-isolator pixel or pixel group amplitudenumber value to the isolator number value is found by subtraction whenusing the equipment of FIG. 13 since both values are obtained aslogarithmic quantities via logarithmic amplifiers 280. Of course, thisratio is only a property of the surface material and condition and angleand is therefore not changed with distance.

FIG. 19 shows a circuit by which the ice/no-ice decision is made foreither a group of pixels or for a single pixel. Inputs to the circuitare the respective non-isolator and isolator mode received signalamplitude values, preferably in logarithmic terms, and the range(distance) from the ice detection apparatus receiver to the surface. Therange signal amplitude value may be that which is available for thepixel or group of pixels under consideration or the average distance tothe surface being scanned (examined) in those cases where localizedrange data are not available. Range data to the center of the fieldunder observation also may be input via an external ultrasonic or laserrangefinder, both of which are commonly available, or via an operatorestimated manual input.

The non-isolator and isolator log amplitude values for the region underconsideration are first corrected for range by adding or subtracting anamount that corresponds to the deviation of the current range from thereference range. (The reference range is that used when the thresholdvalue function or table for the desired set of materials was computed).This correction is performed in the range correction circuits 650 and652, which can be, for example, operational amplifiers if the circuit isof the analog type. If the signal amplitude values are in digital form,digital computation components are used. The term K*LOG(RANGE/REF RANGE)is an offset in the operation of the calculation circuit 652. The valueof K used in circuit 652 is normally equal to "2" but may vary toaccount for any non-linearities or other deviations in the system fromthe ideal model.

The computed corrected non-isolator received signal amplitude is used asthe entry point to a prestored curve fit formula in the thresholdcalculator 660 that approximates the threshold curve 608 of FIG. 18 oras a lookup into a table that has data corresponding to the samefunction. The output of a subtraction circuit 653, which receives thenon-isolator received signal amplitude and the isolator signal amplitudein logarithmic form, is the log of the ratio of optical power detectedin the non-blocking and blocking states, respectively. The thresholdamount, the value of curve 608 at the entry point, is then compared withthe non-isolating/isolating ratio signal (both corrected or bothuncorrected) in the subtraction circuit 654.

The output of subtraction circuit 654, being positive, leads to adeclaration of ice in a decision block circuit 655, since a positiveresult only occurs when the ratio of the non-isolator to isolator signalamplitude is smaller than the threshold value; i.e., the ratio liesbelow the threshold curve 608 of FIG. 18. A negative output fromsubtraction circuit 654 conversely leads to an ice-free decision bydecision block 655, since it is indicative of a ratio greater than thethreshold value.

Although the ratio of non-isolator to isolator received signal amplitudehas been plotted in FIG. 18 as being indicative of an ice or no-icedecision, it is only one of a number of potential evaluation functionsthat can be used to make the ice or no-ice decision. For example,another useful evaluation function may be formed by using as theordinate of the curve the values of the difference of the twologarithmic values (non-isolator and isolator optical state values)divided by the sum of the two logarithmic values and plotting thisfunction for a given set of materials. A simplified plot is shown inFIG. 20. The advantage to using an evaluation function of this type isthat the range of results is restricted to values between "0" and "1"which may then be scaled to any convenient range such as 0-255 for usein digital computation, i.e. 256=2⁸. Negative numbers are not neededbecause the log of the non-isolator amplitude is normally equal to orlarger than the log of the isolator amplitude.

A block diagram of a circuit required to make the ice-no ice decision inaccordance with the evaluation function of FIG. 20 is shown in FIG. 21.This circuit is the same as that of FIG. 19, except that the subtractioncircuit 653 is replaced with a function evaluation circuit 657 for thevalue difference divided by the value sum computation.

The operation of the circuit of FIGS. 19 and 21 are preferably performedin specialized hardware when a very high speed of computation isrequired. Otherwise, they may be performed by a general purposecomputer.

I claim:
 1. A method for detecting on a surface which specularlyreflects light, a presence of a polarization altering substancecomprising the steps of:transmitting light over a transmitting path tosaid surface; receiving said transmitted light over a receiving path forsaid transmitted light from said surface and from said substancealternately in optical isolator and optical nonisolator states;measuring a first intensity of light received in said opticalnon-isolator state; measuring a second intensity of light received insaid optical isolator state; and comparing said first and secondintensities of received light to a plurality of reference data to detectthe presence of said substance on said surface.
 2. A method as in claim1 wherein the steps of measuring said first and second intensities ofreceived light includes determining logarithmic values of said first andsecond intensities of received light.
 3. A method as in claim 1 furthercomprising establishing said reference data by determining a ratio as afunction of said first intensity of light when said surface is a knownsurface with said polarizing substance absent from said known surface.4. A method as in claim 3 wherein the step of establishing saidreference data further comprises determining a threshold value of saidfunction, said threshold value representing a difference betweendifferent conditions of said surface.
 5. A method as in claim 4 whereinone of said conditions of said surface is the presence of one of ice andsnow and another condition an absence thereof.
 6. A method as in claim 3wherein said ratio is said first intensity of received light divided bysaid second intensity of received light, said first and secondintensities of received light being measured when said surface is saidknown surface with said polarizing substance absent from said knownsurface.
 7. A method as in claim 3 wherein said ratio is said secondintensity of received light subtracted from said first intensity ofreceived light divided by said second intensity of received light addedto said first intensity of received light, said first and secondintensities of received light being measured when said surface is saidknown surface with said polarizing substance absent from said knownsurface.
 8. A method as in claim 3 wherein, during the steps formeasuring said first and second intensities of received light, saidtransmitting path and said receiving path are substantially a samelength as a distance from said surface used in establishing saidreference data.
 9. A method as in claim 1 wherein, during the steps formeasuring said first and second intensities of received light, saidtransmitting path and said receiving path are a different length than adistance from said surface at which said reference data wereestablished, and the step of comparing includes correcting for adifference between said distance and said different length.
 10. A methodas in claim 9 wherein the step of comparing includes correcting for saiddifference by adding to said first and second intensities of receivedlight an amount corresponding to said difference.
 11. A method as inclaim 1 further comprising establishing said reference data by measuringsaid first and second intensities of received light when said surface isa known surface with said polarizing substance absent from said knownsurface, and said transmitting path and said receiving path are notparallel to each other.
 12. Apparatus for detecting on a surface whichspecularly reflects light, the presence of a polarization alteringsubstance comprising:means for transmitting light over a transmittingpath to said surface; means for receiving transmitted light; a receivingpath for said transmitted light from said surface and from saidsubstance to said means for receiving; optical means in at least one ofsaid transmitting path and said receiving path; means for alternatingsaid optical means between an optical non-isolator state and an opticalisolator state; said means for receiving including means for measuring afirst intensity of light received at said means for receiving when saidoptical means is in said optical non-isolator state, and means formeasuring a second intensity of light received at said means forreceiving when said optical means is in said optical isolator state; andmeans for comparing said first and second intensities of light to aplurality of reference data to detect the presence of said substance onsaid surface.
 13. Apparatus as in claim 12, wherein said reference dataare in logarithmic form.
 14. Apparatus as in claim 12, furthercomprising means for establishing said reference data by measuring saidfirst and second intensities at a reference distance from said surfacewhen said surface is a known surface with said polarization alteringsubstance absent from said known surface.
 15. Apparatus as in claim 14wherein said means for establishing said reference data furthercomprises;means for calculating a first ratio of said first intensity tosaid second intensity and making a first comparison of said first ratioto said first intensity when said polarization altering substance isabsent from said known surface; means for calculating a second ratio ofsaid first intensity to said second intensity and making a secondcomparison of said second ratio to said first intensity when saidpolarization altering substance is present on said surface; and meansfor determining a threshold to establish a difference between said firstand second comparisons representing different conditions of the surfacebeing investigated.
 16. Apparatus as in claim 15 wherein one of saidconditions of said surface is the presence of one of ice and snow andanother condition an absence thereof.
 17. Apparatus as in claim 14wherein:said transmitting path and said receiving path are substantiallya same length as said reference distance used by said means forestablishing reference data.
 18. Apparatus as in claim 14 wherein saidtransmitting path and said receiving path are substantially a differentlength from said reference distance and said means for comparingincludes means for correcting for said different length.
 19. Apparatusas in claim 14 wherein said means for establishing said reference dataincludes means for measuring said first and second intensities when saidsurface is a known surface with said polarization altering substanceabsent from said known surface at a plurality of angles with respect toa normal of said surface.
 20. Apparatus as in claim 14 wherein saidmeans for establishing said reference data includes means for measuringsaid first and second intensities at a plurality of angles with respectto a normal of said surface when said surface is a known surface withsaid polarization altering substance present.
 21. Apparatus as in claim12 wherein:said means for transmitting light includes only onetransmitter; and said means for receiving transmitted light includes atleast one receiver.
 22. Apparatus as in claim 12 wherein:said means fortransmitting light includes at least one transmitter; and said means forreceiving transmitted light includes only one receiver.
 23. Apparatus asin claim 12 wherein:said means for transmitting light includes at leastone transmitter; and said means for receiving transmitted light includesat least one receiver.
 24. Apparatus for detecting on a surface whichspecularly reflects light, the presence of a polarization alteringsubstance comprising:means for transmitting light over a transmittingpath to said surface; means for receiving transmitted light; a receivingpath for said transmitted light from said surface and from saidsubstance to said means for receiving; optical means in at least one ofsaid transmitting path and said receiving path; means for alternatingsaid optical means between an optical non-isolator state and an opticalisolator state; means for measuring a first intensity of light receivedat said means for receiving when said optical means is in said opticalnon-isolator state and outputting a first logarithmic signal; means formeasuring a second intensity of light received at said means forreceiving when said optical means is in said optical isolator state andoutputting a second logarithmic signal; means, responsive to a length ofsaid receiving path, for range-correcting said first and secondlogarithmic signals; means for comparing said range-corrected secondlogarithmic signal to said range-corrected first logarithmic signal andoutputting a ratio signal; means for calculating a threshold signal; andmeans for comparing said ratio signal to said threshold signal. 25.Apparatus according to claim 24, wherein:said means for comparing saidrange-corrected second logarithmic signal to said range-corrected firstlogarithmic signal includes a first subtraction circuit; and said meansfor comparing said ratio signal to said threshold signal includes asecond subtraction circuit.
 26. Apparatus according to claim 24, whereinsaid ratio signal represents a ratio of said second range-correctedlogarithmic signal subtracted from said first range-correctedlogarithmic signal to said second range-corrected logarithmic signaladded to first range-corrected logarithmic signal.